Nanoscale Spintronic Chemical Sensor

ABSTRACT

In general, the present disclosure is directed toward a novel hybrid spintronic device for converting chemical absorption into a change in magnetoresistance. This device uses a novel magnetic material which depends on the attachment of an organic structure to a metallic film for its magnetism. Changes in the chemical environment lead to absorption on the surface of this organometallic bilayer and thus modify its magnetic properties. The change in magnetic properties, in turn, leads to a change in the resistance of a magnetoresistive structure or a spin transistor structure, allowing a standard electrical detection of the chemical change in the sensor surface.

PRIORITY INFORMATION

This present application is a divisional of and claims priority to U.S.patent application Ser. No. 13/732,823 titled “Nanoscale SpintronicChemical Sensor” of Crawford, et al. filed on Jan. 2, 2013; and is adivisional of and claims priority to U.S. patent application Ser. No.12/442,951 titled “Nanoscale Spintronic Chemical Sensor” of Crawford, etal. filed on Jan. 4, 2010, which is a nationalization phase applicationof International Application No. PCT/US2007/081070 filed on Oct. 11,2007; and claims priority to the U.S. Provisional Patent Application No.60/851,091 filed on Oct. 11, 2006, titled “Nanoscale Spintronic ChemicalSensor” of Crawford, et al., the disclosures of which are incorporatedby reference herein.

BACKGROUND OF INVENTION

Chemical sensors which are lightweight, portable, and inexpensive areneeded for anti-terrorism and environmental applications.Magnetoresistive sensors have found applications in magnetic datastorage as well as in automotive and other process control sensingapplications. However, converting chemical sensing into a magneticallydetectable signal has not been reported. The ability to detectenvironmental contaminants through low-voltage, battery or solar-poweredmagnetoresistive sensors would be a major breakthrough in deploying suchsensors in a variety of areas, as opposed to optical chemical sensingwhich requires both an optical source such as a laser (often intense forlaser-induced breakdown spectroscopy) and a detector.

As such, a need exists for a low-voltage, battery or solar-poweredmagnetoresistive sensor having the ability to detect environmentalcontaminants.

SUMMARY OF INVENTION

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In general, the present disclosure is directed toward a novel hybridspintronic device for converting chemical absorption into a change inmagnetoresistance. This device uses a novel magnetic material whichdepends on the attachment of an organic structure to a metallic film forits magnetism. Changes in the chemical environment lead to absorption onthe surface of this organometallic bilayer and thus modify its magneticproperties. The change in magnetic properties, in turn, leads to achange in the resistance of a magnetoresistive structure or a spintransistor structure, allowing a standard electrical detection of thechemical change in the sensor surface.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, which includesreference to the accompanying figures, in which:

FIGS. 1A and 1B show a schematic of one embodiment of a sensor having athin bar of an organically capped metal layer patterned on top offerromagnetic injector and detector films.

FIGS. 2A-2C show three different embodiments of a device for sensing thepresence of a chemical in the atmosphere.

FIGS. 2D-2F show a plot of the spin current vs. time of the devicesshown in FIGS. 2A-2C, respectively.

FIGS. 3A-3B show how the resistance of the devices in FIGS. 1-2 changeswith an applied magnetic field.

FIG. 4 shows another embodiment of the spintronic chemical sensor.

FIG. 5 shows another embodiment of a sensor, this time a magnetic tunneljunction (MTJ) device geometry.

FIG. 6 shows yet another embodiment, wherein instead of using the thiolSAM as the tunnel barrier, a conventional aluminum-oxide (Al₂O₃) or MgOor other tunnel barrier is used.

FIG. 7 shows a magnetic tunnel junction sensor employing an organicallycapped metal layer as a spin dependent tunnel barrier.

FIG. 8 shows Plot 1 in which the upper graph shows hysteresis loops forAu, Au capped with dodecane thiol (1 mM for 24 hours), and the samplemount (top); and the lower graph is the difference between the Au andAu-thiol loops.

FIG. 9 shows Plot a and b, which show resistance as a function of fieldfor two different Co (20 nm)-Au (5 nm) and Co (20)-Au (5)-thiol (1 mMdodecane for ½ hour) films.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION OF INVENTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

The present invention is generally directed to a novel spintronic sensortechnology which can be employed as a chemical sensor for chemicaldetection, a magnetic field sensor for magnetic data storageapplications (e.g. mass storage or magnetoresistive random accessmemory), and as a spin transistor. This invention uses achemically-controlled magnetic material as a component of a multilayerdevice together with standard ferromagnetic layers to offer chemicalsensitivity, as well as enhanced magnetic sensitivity compared withtraditional magnetoresistive sensors.

In one particular embodiment, the bi-layer sensor includes anorganically-capped metal (e.g., gold) film which displays permanentmagnetism as a system. For example, sulfur atom linkages (i.e., thiollinkages) can be used to bond the organic molecule to the metal filmcreating a multilayer film (e.g., a bilayer, trilayer, etc.), such as ina thiol self-assembled monolayer or thiol SAM, or polyalinine. However,many other organic structures that exhibit permanent magnetism can alsobe used to cap the films. Moreover, while gold (Au) is a preferredmetal, other metals, such as silver (Ag), aluminum (Al), copper (Cu),platinum (Pt), palladium (Pd), etc., can exhibit similar properties asAu when used in devices of the present invention. In particular, Ag, Al,and Cu all have an odd number of electrons, as does Au. It is proposedby the present invention that these metallic-organic (e.g. Au-thiol)layers be used in conjunction with ferromagnetic thin films as the basisof a novel class of magnetoresistive (spintronic) devices.

In one embodiment of the present invention, an organically cappedbilayer film (e.g., Au-thiol) in a spintronic device is used as aspintronic chemical sensor. It has been reported that the magneticmoment in a Au-polyalinine bilayer (with a sulfur, i.e. thiol, linkeratom to the Au) decreases with time after being exposed to air. Thus,the present inventors have discovered that the magnetic resistance ofthe Au-polyalinine bilayer can be used in a chemical sensor to detectthe presence of an analyte in the atmosphere. Specifically, any analytethat can attach (e.g., bond) to the organic cap can create a change inthe magnetic resistance of the bilayer.

For example, FIG. 1 shows an exemplary schematic of one embodiment ofthe proposed sensor 1. A thin bar or film 10 of metal (e.g., Au) ispatterned on top of ferromagnetic injector film 12 and ferromagneticdetector film 14. These structures are arranged in a geometry which iscommonly used to measure spin diffusion. For example, the metal film 10capped with organic tails 11 attached to the metal 10 through sulfurlinkages 13 is positioned over a biasing magnet 15 and between theferromagnetic injector film 12 and ferromagnetic detector film 14. Themetal film 10 capped with organic tails 11 can be referred to as abilayer film. A charge current enters the ferromagnet injector 12 andpasses to ground through the organically capped metal film 10 through anOhmic or tunnel contact. The spins of the electrons entering theorganically capped film 10 are polarized by passing through theferromagnetic injector 12. Therefore, while the charge current isreturned to ground, there is a characteristic length over which thespins diffuse into the metal film 10, called the spin diffusion length(SDL) shown as L_(SDL) in FIG. 1. In Au, different measurements of theSDL suggest that the SDL ranges from 50-250 nm. This length L_(SDL) setsthe active area of the device. If the Au film is shorter than the SDL,then the spins are detected as a spin current in the ferromagneticdetector.

FIGS. 2 a-2 c show three different types of devices. The first device 2,shown in FIG. 2 a, has a metal film 10′ without any organic layer(thiol, polyalinine, etc.). The spin current is constant with time for agiven charge current. However, if instead of the bare metal film 10′, anorganically capped metal bilayer 10 (e.g., Au-thiol with polyalinine) isemployed, such as in FIG. 2 b, and if the system 3 is kept in an inertatmosphere, no spin current will be detected, because the finitemagnetization of the organically capped metal bilayer 10 randomizes thespin before it has a chance to diffuse across the region. Once thisdevice is exposed to the atmosphere (as shown in FIG. 2 c), an analyte15 can be attached to the organic tail 11 of the bilayer 10, and themagnetism decreases with time. Thus, the spin diffusion current can bedetected again. These three embodiments are shown in the schematicgraphs on the right side of the page, where the hypothetical spincurrent of the three cases is graphed as a function of time. Over time,in the atmospheric embodiment 4, the signal changes as the magnetism isreduced. By modifying the organic tail 11 of the bilayer 10, differentsensitivities to different atmospheric chemicals (e.g., analytes 15) canbe engineered, which is within the capabilities of one of ordinary skillin the art. As a result, an array of these spintronic devices could beused to sense a multitude of atmospheric chemicals. For example, adevice can be engineered having a specific organic tail 11 that has astrong affinity for a particular analyte 15. After the device has beenconstructed, it can be exposed to the particular analyte 15 to determinethe affect such an exposure has on the device. Then, when in use, thesensor can be used to detect the presence of that targeted analyte 15 bymonitoring for that particular response predetermined by the controldevice.

FIG. 3 shows how the resistance of the devices in FIGS. 1-2 changes withan applied magnetic field. The graph on the left shows a hystereticchange in resistance as the spin current is overcome by the appliedmagnetic field. The graph on the right, where the Au-thiol layer ismagnetic, shows no change in resistance. The intermediate states betweenthese two graphs are what this sensor would detect as the Au-thiolmagnetism changes.

FIG. 4 shows an alternative embodiment of a spintronic chemical sensor.This device geometry is similar to a current-in-plane (CIP) giantmagnetoresistance multilayer or spin valve. Here the multilayerstructure 5 has an antiferromagnetic layer 16 (e.g. PtMn or IrMn) to pinthe magnetization of a ferromagnetic layer 18 (e.g., CoFe or NiFealloy). A spacer layer 20 (e.g., Cu) is deposited, and followed finallyby the organically capped metal bilayer structure 10. Permanent magnets22 can be deposited on the sides of the structure 5 to bias the layersinto a non-aligned state, and leads 24 a, 24 b are deposited on thesides such that an electric current flows sideways through themultilayer. The active area (i.e., the organic tails 11) of the deviceis exposed to the atmosphere, and the magnitude of the observedmagnetoresistance will depend on the degree to which the organicallycapped metal bilayer 10 is magnetic. As the magnetization decreases dueto adsorption of atmospheric chemicals (i.e., analytes 15), themagnetoresistance will correspondingly decrease, enabling one to monitorthe atmosphere for particular chemical species.

FIG. 5 shows another embodiment of the present invention, this time thesensor device 6 has a magnetic tunnel junction (MTJ) device geometry.Such structures have been demonstrated to have magnetoresistance whenused with ferromagnetic electrodes. Here the current flows perpendicularto the plane (CPP). A metal-thiol-metal trilayer (e.g., Au-thiol-Autrilayer), where the sulfurs 13 are located on top 32 of the structureas well as the bottom 34, with the organic tails 11 being the insulatingtunnel barrier. The top lead film 36 is extremely thin so as to changeits magnetism when atmospheric chemicals adsorb onto the lead.Additionally, a ferromagnetic layer 38 and an antiferromagnetic layer 40are present. A top lead 42 and a bottom lead 44 supply a current throughthe system.

FIG. 6 shows another embodiment of a MTJ device 6′, wherein instead ofusing the thiol SAM as the tunnel barrier, a conventional aluminum-oxide(Al₂O₃) or MgO or other tunnel barrier 50 is used, only in this case,the top lead is made of an organically capped metal film 10, i.e. thetop electrode is treated with a layer of thiols to facilitate itsability to sense atmospheric chemicals. In this embodiment, it is thecurrent through the top lead whose degree of spin alignment with thepinned ferromagnet determines the resistance of the structure andchanges under atmospheric conditions.

Finally, FIG. 7 shows a magnetic tunnel junction sensor 6″ employing anorganically capped metal bilayer 10 (e.g., Au-thiol) as a spin dependenttunnel barrier. In FIG. 7, two ferromagnetic layers 38 a, 38 b arepresent as well as the organically capped metal layer 10. To the extentthat having spin dependent transport through the organically cappedmetal layer 10 enhances the magnetoresistance, this device could be usedas an improved magnetic field sensor, for example, in next generationmagnetic random access memory devices or hard disc drives for sensingbit directions via magnetic fields, or in any other magnetoresistivesensor technology. FIG. 7 could be modified to have a single ferromagnettogether with the organically capped metal layer 10 instead of twoferromagnetic layers. One skilled in the art would realize that thereare numerous other combinations of conventional magnetoresistive devicegeometries wherein organically capped metal layer 10 magnetic layerscould be employed.

The present invention proposes candidate sensor designs for chemical andmagnetic field sensing, employing a metallic-organic (eg. Au-thiol)spintronic bilayer as a spin-active electron transport element, which issensitive to atmospheric conditions and can be chemically adjusted tohave different sensitivities to different chemical species. An array ofthese devices could be fabricated similarly to an MRAM device, but whereindividual sets of devices would sense a different chemical species.Such a device would be a low power, portable alternative to otherproposed atmospheric chemical sensor technologies.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood the aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in the appended claims.

EXAMPLES

Plot 1 shows preliminary magnetometry data we have taken for a 24 hrAu/dodecanethiol (1 mM in ethanol) film, a Au film control, a samplemount control, and the difference between the Au-thiol and Au controlloops. The samples are 1×1 mm² photolithographically patterned Ausquares grown on Si wafers. The observed magnetization is˜7.5×10^(−4 emu/cm) ² at 1 T, 2.7× lower than the 2×10⁻³ emu/cm²observed by Hernando et al. for Lewis conjugate on glass, but 8× largerthan the 9.33×10⁻⁵ emu/cm² found by for polyalanine on glass. Takentogether, these results display three different magnetizations for threedifferent Au-S chemistries. However, achieving reproducibility has beenchallenging. We have fabricated samples which, while showing weakermoments, display increasing moment with immersion time. Other samplesdisplay para- and diamagnetic moments near the sensitivity limit ofmagnetization values highlights the need for meticulous control duringsample fabrication, with a focus on which process variables directlyinfluence the Au-thiol moment. For example, Au hydrophilicity andcleanliness, as well as the Au crystalline texture and roughness maystrongly affect the magnetization.

Plot 1—The upper graph shows hysteresis loops for Au, Au capped withdodecane thiol (1 mM for 24 hours), and the sample mount (top). Thelower graph is the difference between the Au and Au-thiol loops.

An example of MR detection of Au-thiol magnetism is shown in plots 2A-B,where the resistance of two different Co/Au (red) and Co/Au/thiol (blue)thin films is plotted as of our vibrating sample magnetometer (VSM),i.e. much weaker magnetization. The wide range a function ofperpendicular field. In both cases the two curves have differenttransition fields for the in-plane MR component (features between+/−1000 Oe), and a 30 minute immersion in 1 mM dodecane thiol modifiesthe field necessary to observe the in-plane transitions in the Co filmsby 500 and 200 Oe for Plots 2A and 2B respectively. Repeated experimentsshow similar shifts of varying sizes, and an immersion in pure ethanolshows no shift within experimental error. We have observed subtractiveand additive field shifts, as well as an asymmetry in the field shiftrate as a function of sample angle relative to the applied field.Au-thiol magnetism appears to modify the magnetotransport in Co films byproducing a bias field which either opposes or assists the appliedfield. This shift changes with time after immersion, suggesting that thethiol SAM evolves with time. Plot 2 a and 2 b—Resistance as a functionof field for two different Co (20 nm)-Au (5 nm) and Co (20)-Au (5)-thiol(1 mM dodecane for ½ hour) films. The peaks are the in-plane AMRresponse for out-of-plane fields which are not perfectly normal to thefilm plane. The adsorption of thiol shifts these peaks to lower fieldsin these cases, along with other changes.

Those of ordinary skill in the art will understand that the devicesdisclosed and described herein are meant only as examples and are notintended to limit the scope of the invention.

1. A spintronic chemical sensor having a magnetic tunnel junction devicegeometry, the sensor comprising: a metal-thiol-metal trilayer defining atop and a bottom, wherein sulfur atoms are located on top of thestructure as well as the bottom, wherein organic tails act as aninsulating tunnel barrier between the metal on the top and the metal onthe bottom; a top lead film adjacent to the top of the metal-thiol-metaltrilayer; a ferromagnet layer adjacent to the bottom of themetal-thiol-metal trilayer; and an antiferromagnetic layer adjacent tothe ferromagnet layer.
 2. The spintronic chemical sensor as in claim 1,wherein the metal-thiol-metal trilayer comprises an organic layerpositioned between two metal films, and wherein the organic layer isbonded to the metal film via a thiol linkage.
 3. The spintronic chemicalsensor as in claim 1, wherein the metal-thiol-metal trilayer comprisesan organic layer positioned between two metal films, and wherein theorganic layer comprises a polyalinine.
 4. The spintronic chemical sensoras in claim 3, wherein the metal films comprise gold.
 5. The spintronicchemical sensor as in claim 3, wherein the metal films comprise silver.6. The spintronic chemical sensor as in claim 1, wherein themetal-thiol-metal trilayer has a spin diffusion length of from about 50nanometers to about 250 nanometers.
 7. A spintronic chemical sensorhaving a magnetic tunnel junction device geometry, the sensorcomprising: multilayer sensor comprising an organically capped metalfilm, wherein the multilayer sensor displays permanent magnetism, themultilayer sensor having a top and a bottom; a first ferromagnet layeradjacent to the top of the multilayer sensor; a second ferromagnet layeradjacent to the bottom of the multilayer sensor; and anantiferromagnetic layer adjacent to the second ferromagnet layer.
 8. Thespintronic chemical sensor as in claim 7, wherein the organically cappedmetal film comprises an organic layer and at least one metal film, andwherein the organic layer is bonded to the metal film via a thiollinkage.
 9. The spintronic chemical sensor as in claim 7, wherein theorganically capped metal film comprises an organic layer and at leastone metal film, and wherein the organic layer comprises a polyalinine.10. The spintronic chemical sensor as in claim 9, wherein the metalfilms comprise gold.
 11. The spintronic chemical sensor as in claim 9,wherein the metal films comprise silver.
 12. The spintronic chemicalsensor as in claim 7, wherein the organically capped metal film has aspin diffusion length of from about 50 nanometers to about 250nanometers.